The long term goal of this project is to understand the mechanism that underlies differentiation and cell cycle control during terminal differentiation of B cells to plasma cells. In vivo, terminal differentiation of B cells is characterized by increases in Ig synthesis and secretion, reduction in surface Ig and MHC class H expression, morphological maturation and cell cycle arrest. Despite this knowledge, the underlying mechanism is not well understood. Interluekin-6 (IL-6) has a physiologic role in late stage B cell differentiation, as evident by the development of plasmacytosis in transgenic mice overexpressing IL-6 and by the deficiencies in secondary Ig responses in IL-6-deficient mice. We have shown that stimulation of human B lymphoblastoid cells with IL-6 in vitro recapitulates the major hallmarks of B cell terminal differentiation in vivo. The objective of this proposal is to elucidate the mechanism by which IL-6 signals are transduced to regulate Ig synthesis in B cells. The IL-6 signals are thought to be transduced by two pathways: the rapid and transient JakStat pathway involving activation of the latent transcription factors Stat3 and Statl, and a more stable NF-IL6 pathway involying the basic- leucine zipper transcription factor NF-IL6. Despite this wealth of information, two crucial issues remain unresolved. One concerns the relationship between the two pathways and the other the determination of the promoter specificity in each pathway. Based on our preliminary studies, we hypothesize that the transient Jak-Stat pathway and the stable NF-IL6 pathways are functionally coupled for physiologic responses to IL-6 by sequential activation of NF-IL6, which activates and inhibits downstream genes according to the ratio of NF-IL6 isoforms and by dimerization between NF-IL6 and Jun. To test this hypothesis, we propose to (l) elucidate the coupling between the Jak-Stat pathway and the NF-IL6 pathway, by determining the requirement of Stat3 and Statl and serine phosphorylation for the immediate activation of the NF-IL-6 promoter, (2) determine the roles of NF-IL6 and Jun in stable IL-6 signaling, and (3) investigate the physiologic roles of NF-IL6 isoforms in the regulation of Ig - synthesis by studying the activation and inhibition of Ig promoters by NF-IL6 isoforms in vitro and in vivo. These studies should provide significant insight into the the mechanisms that underlie cytokine signaling in the immune system and terminal differentiation of B cells.